Usually, a four-stroke cycle engine to be mounted upon a vehicle such as an automobile and a motorcycle is provided with intake and exhaust valves at a position above its combustion chamber and these valves are driven by means of a valve actuating mechanism. Specifically, the valve actuating mechanism is provided with a crank shaft of the engine so that the intake and exhaust valves are caused to move in an up and down reciprocating mode in accordance with a predetermined timing operation by means of a cam which is formed upon a cam shaft.
It is desirable in connection with a four-stroke cycle engine that a high output may be obtained throughout a broad speed range extending from a low speed region to an intermediate-high speed region, that is, that the power band is wide.
However, in a conventional valve actuating mechanism, since the timing for opening or closing a valve and the amount of lift are fixed, only an output characteristic having a peak value within a specific engine speed region may be obtained and one is therefore forced to make a choice as to whether the output characteristic in the low speed region is to be emphasized or the output characteristic in the intermediate-high speed region is to be emphasized.